Electrically heated thermoresponsive switch



fIgrBf Oct. 1947- G. c. ARMSTRONG 2,428,539

ELECTRICALLY HEATED THERMORESPOIiSIVE SWITCH Filed June 9, 1944 INVENTORWITNESSES: Z5 Geo/ye C. flrmsfrany:

ATTORNEY aa' m load protective means whose synchronous Patented Oct. '1,1941 ELECTRICALLY HEATED THEBMOBESPON- SIYE George G. Armstrong, toWestinghouse E Forest Hills, Po... assignmiectrle Corporation, Eastinnia Pittsburgh, Pa a corporation of Penna! Application June a, 1944,Serial no. 539,495 a claims. (cl. "-2) My invention relates toelectrically heated,

thermoresponsive switches, such as overload reits aspects to relays forprotecting alternating-current motors from overlays, and in one of loadsoccurring during starting or accelerating periods.

It is an object of my invention to provide overtime constant is capableof being adjusted to lower values than those usually obtainable inthermally controlled overload relays of the types now available.

Another object oi my invention aims at providing a thermal relay for theprotection of synchronous and wound rotor motors that changes its timeconstant or operating characteristic in dependence upon the frequency ofthe secondary motor current so locked rotor protection while remaininginoperative when the motor accelerates from zero to synchronous speedunder normal or permissible starting conditions.

The significance of these objects will be appreelated from aconsideration of an example relating tothe protection of the damperwindings of motors during starting periods. In order to obtain such aprotection, it is known to connect an overload relay in the fieldcircuit of the motor, for instance, in series with the dischargeresistor while the latter is connected across the rotor winding duringthe starting periods of the motor. When the motor accelerates from zerospeed, a secondary current circulates in the field and relay circuithaving at first the frequency of the line current. As the motorincreases its speed, the frequency decreases until it becomes zero atsynchronous speed. The relay, energized in response to the secondarycurrent of changing frequency, must trip quickly it the motor fails tostart, but not trip it the motor ac-' celerates properly. In order tosecure such-a frequency-dependent operation, a saturable reactor hasbeen connected in parallel to the thermal overload relay. ance inresponse to changes in frequency, this shunt reactor causes the relay tobe more strongly energized at high frequency and to receive lessenergization at lower frequency. However, the application of such orsimilar means is unsatisfactory in many cases of application, becausethe available thermal overload relays have too long a time constant ascompared with the time characteristic of the motor starting'operation.It has also been difiicult in such relay-reactor combinations to obtaina .sutlicient differential in time for aflording a satisfactorylockedmotor protec- Due to its changes in reactthat the relay affordsreliable in the saturable shunt reactor.

tion together with a satisfactory safety margin which permits the motorto come to speed under normal or acceptable operating conditions. Thisshortcoming is due partly to the thermal inertia of the designed topermit a relatively long starting period. In order to obtain. underthese conditions, a trip time anddanger protection, heater temperatureshave to be put up with which may come dangerously near the burn outpoint. Therefore, the known thermal means for protecting damper windingsor synchronousmotors are in many cases rather difficult to adjust forsafe locked rotor protection without unduly limiting the accelerationcharacteristic. Hence, in this field of application, the. above-statedobjects of the invention involve the aim to secure a safelocked rotorprotection, while affording at the same time a sufilciently wide rangeof acceleration which permits starting the motor under diilerent loadswithout causing the relay to trip unnecessarily under acceleratingconditions-not dangerous to the motor.

Another object of the invention-is to devise a thermal relay or thelatched-in type whose thermally controlled latching and releasingmechanism is of especially simple design and small size,

but has nevertheless an accurate releasing operation and a safe latchingoperation even when exposed to vibration.

in order to achieve provides a thermal. relay in which a shaft member istraversed by electric current andconsists of resistance material so asto be heated in accordance with the load attached thereto. This shaftmember serves as a seat for a clutching spring winding which engages theshaft frictionally and thereby prevents a spring-biasedcontact-controlling member from moving under its bias. The clutchingspring consists preferably of a expands its turns radially when heated,thereby releasing its frictional grip on the shaft member.

These and other features or the invention will be more fully understoodfrom the following description in conjunction with the appertaining P rton of the same relay,

available relays, and partly to the RI drop The relays areshunt effectas required for these objects, my invention conditions of the circuithelical bimetal strip and view of a relay according. to the invention,especially the sectionbeing taken,

- 3 along a plane indicated in Fig. 1 bythe dot and dash line marked11-11;

Fig. 3 is a front view of the complete relay according to Figs. 1 audit;

Fig. 4 is an axial cross-section through the clutching and releasingdevice of the relay shown on a scale larger than that of the precedingfigures;

Fig. 5 illustrates the movable contact member of the same relay;

Fig, 6 is a circuit diagram of a synchronous motor provided with aprotective relay according to the invention; and

Fig. 7 shows a shaft pertaining to the relay of Figs. 1 to 5,representing a slightly tapered portion of the shaft in an exaggeratedmanner for the purpose of explanation.

Referring to Figs. 1 through 5, numeral I denotes the magnetic structureof a transformer provided with a primary winding 2 and a secondarywinding 3. A pair of supporting bars 4 and 5 is rigidly attached to thestructure I at each side thereof. These supporting bars carry aninsulating base 6 which is fastened thereto by means of screws 1. Asupporting block 8 01' highly conductive material is mounted on base 6by means of a screw 9 which connects one terminal of the primary winding3 to block 8. A metal block I is press-fitted into a bore of block 8,thereby securing a sleeve II in proper position. This sleeve II isrotatable relative to the block 8 and plug III and forms a hub for amovable contact member I2 with which it is firmly connected by means ofa collar I3 and asnap-ring I4.

A shaft member I passes through the center bore of plug I0 and throughsleeve II and is axially displaceable within plug I0. A screw 16 permitssecuring shaft I5 in a selected axial position relative to plug .IO. Theshaft member I5 consists of non-corrosive resistance material, such asnickel-chromium, and has an axial bore in order to assume the highelectric resistance necessary for heating purposes. The end ll of shaftI5 forms a binding post and is connected with the other terminal of thesecondary winding 3. Hence, when in operation, the current induced inthe secondary coil 3 passes from screw 9 through block 8, plug I0, andshaft I5 to the terminal I'I. Thus, the shaft I5 forms a heater whosetemperature depends on the secondary transformer current.

The shaft member I5 is slightly tapered from about its axial midpointtoward the binding post IT as is apparent from the exaggerated showingof the tapering in Fig. 7 A helical bimetal strip I8 is seated on thetapered portion of the shaft member and has an end I9 in engagement withthe axial slot 20 of sleeve II. The other end of the bimetal strip I8 isfree, but the diameter of the winding turns is so dimensioned that thewinding grips the shaft I5 frictionally when cold, and hence prevents arotation of the sleeve II relative to the block 8 in the winding-up ortightening direction of the bimetal winding. This frictional grip isloosened when the sleeve I I is rotated in the opposite direction of thewinding turns because such rotation will widen the turns and henceloosen the frictional engagement between the winding and the shaft.

The contact member I2 has a projection 2I which forms an eye for holdingone end of a biasing spring 22 whose other end is fastened to a sleeve3|. This sleeve permits adjusting the tension of spring 22 by screwingit more or less into base 0. thereby displacing the position of theappertaining fastening point of the spring 22. The contact member I2 isfurther provided with a knife-type contact portion 22 which serves as abreak" contact and cooperates with a stationary contact 24 mounted onbase 2. Contact 24 comprises a leaf spring and has a slight frictionalgrip on the movable contact when the latter is in the illustratedcontact closing position. A lug 25 of contact member I2 serves as a"make" contact and cooperates with a stationary contact 26 of base 8. Athreaded projection 21 01 contact member I2 carries an actuatin button28. A screw 29 on top of block 8 forms a terminal for connecting themovable contact member I2 through the elements I I, I8 and I with oneterminal of the circuit to be controlled by the relay, while the otherterminals of this circuit are to be attached to the binding screws ofstationary contacts 24 and 28.

When the movable contact member I2 is in the operative positionillustrated in Figs, 1, 2 and 3, the break contact between elements 23and 24 is closed, and the make" contact between elements 25 and 26 isopen. The biasing spring 22 tends to rotate contact member I2 clockwise,but such rotation is prevented by the frictional grip of the bimetalwinding I8 on the relay shaft I5. When the secondary winding 3 of thetransformer supplies sufficient current to heat the shaft member I5, thefrictional grip of the bimetal winding is somewhat loosened and, in thecase of an overload, the frictional clutch is released so that hub IIand contact member I2 are free to rotate under the bias of spring 22.Due to the frictional grip of stationary contact 24 on the movable breakportion 23 of contact member I 2, the motion starts only after thefrictional grip of the bimetal winding becomes less than the difference'betweenthe torque caused by spring 22 and the frictional torque causedby the contact 24. When the break contacts separate, the spring torquebecomes definitely superior. so that the interruption of the "breakcontacts occurs under snap action,

The relay can be set by pushing button 28 toward base 8 into theillustrated position, provided the bimetal winding has cooledsufficiently. Since this return motion tends to unwind the turns of thebimetal winding, the latter slides about shaft I5 until the circuitclosing position of contacts 23 and 24 is reached. Upon release ofbutton 28, the frictional clutch grip is again effective as describedabove.

Considering the relay mechanism as such, it wil1 be seen from theforegoin description that the device is of especially simple design dueto the fact that the member I 5 forms not only the shaft of the relaybut is also an immediate part of the releasable clutch as well as thecurrent responsive heater for controlling the release operation.

Due to the tapering design of the clutching portion of the shaft, therelay can be calibrated by fastening the shaft I5 in different axialpositions relative to plug I0.

Due to the direct heating of shaft I5 by electric current to whoseintensity the relay is to respond, a very accurate tripping operation isachieved, and it is possible to adjust the shaft member for an extremelylow time constant of thermal releasing operation. The frictional gripbetween the shaft and the bimetal strip extends around the entireperiphery of the shaft and over a multiplicity of turns and, hence, isstrong enough to contactor C, the

' heater 1! is not suiilcient to trip current in a protect the videdwith damper windings and has an auxili field winding W connected to sliprings whose corresponding brushes are connected to a switch 8. Duringstarting periods and after the closure of switch S connects the fieldwindin series with the primary winding 2 of the transformer T, whichforms part of a protective relay unit R designed in accordance withFigs. 1 through 5. when the armature has reached synchronous speed, theswitch S is actuated and connects a direct-current source, heresymbolized by a battery E, across winding W.

During the starting period, 1. e. when the dis charge resistor D and thetransformer P y 1 are series-connected across the winding W, the currentflowing in this secondary motor circuit high starting frequency whichthe armature comes up to full induced in the secondary winding 3 of '1whose frequency is also high celerating period and current vanishes atsynchronous speed. Since the transformer core saturates rapidly as thefrequency decreases, the current passing from the secondary transformerwinding the accelerating period. If the rotor the initial startingfrequency of line frequency persists. In this case, the tripping time ofthe relay is extremely short by virtue of-the direct heating of theshaft and clutch part as explained previously. s

As described in the foregoing, the proper time constant at startingfrequencies can be adiusted readily by displacing the relay shaft 15.

The release of the relay clutch and the interruption of the breakcontacts 23 and it caused thereby have the effect of deenergizing theline contactor C, so disconnected from the line terminals.-

declines gradually until the hereto.

6 my invention as defined in the claims appended I claim as myinvention:

1. A thermoresponsive device comprising a support, an elongated heatingelement mounted on said support so as to be axially displaceablerelative to said support and having a conical portion,

Y a thermoresponsive radially expansible clutching winding seated onsaid conical portion for frictionally gripping it when cold, said mentand said winding alone constituting a clutching and unclutching device,a movable control member biased for motion in a given direction andconnected'with said winding to be normally prevented thereby from movingunder its bias, and means connected to said heating ele- .ment forpassing current therethrough in order 3 through the A the relay duringis locked,

of the relay are not illustrated in Hg. 6. They that while I haveillusdesigned and favorable for protecting alternating-current motors,by an operation depending upon the frequency of the secondary motorcircuit, it will be obvious that the thermal relay mechanism can also beused for a transformer.

skilled in the art. upon studying the present dis closure, thatvariousmodifications can be made do the essential featin'es of to release saidwinding at given current condition as calibrated by the position of saiddisplaceable heating element relative to said support.

2. A thermoresponsive device comprising a support, a conical shaftmember of conductive resistance material mounted on said support andbeing axially displaceable relative thereto, a radially expansiblebimetal winding seated on said member for frictionally gripping it whencold, said shaft member and said winding alone constituting a clutchingand unclutching device, a movable control mem r biased for motion in agiven direction and connected with said winding to be normally preventedthereby from moving under its bias, and circuit means connected to saidshaft member for passing current therethrough in order to release saidwinding at given current conditions.

3. A thermoresponsive device comprising, a support, a hollow andexteriorly conical shaft member mounted on said support and displaceableaxially with respect thereto, a helical and radially thermo-expansibleclutching spring seated on said conical member for frictionally grippingit when cold, said shaft member and said spring alone constituting aclutching and unclutching device, a device having a movable memberrotatable about the axis of said shaft member and being biased formotion in a given direction and connected with said winding to benormally prevented thereby from moving under its bias, and circuit meansconnected to said shaft member for passing current therethrough in orderto release said winding at given current conditions.

GEORGE NG.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS heating ele-

